• Cell-mediated hypersensitivity (CMH) to house dust mite antigens, as determined by increased DNA synthesis after coculture of mononuclear cells with Dermatophagoides farinae extract, was demonstrated in 11 of 16 children with atopic dermatitis (AD) as compared with two of 14 control subjects. Patient and control responses peaked at the same antigen dose (12.4 μg of mite protein per 105 cells). Patient responses were significantly greater at all mite protein concentrations. The increased DNA synthesis of stimulated cells was demonstrated to be a T-cell response in the one patient in whom T-cell-enriched cultures were studied. There was no relationship between CMH and IgE-mediated hypersensitivity to D farinae in the patient group. It is suggested that AD is associated with CMH reactions to allergens and that antigen competition may be involved in the reduced CMH responses to mitogens and antigens noted by other investigators.